Method for determining the temperature downstream the entry of a catalytic converter for a turbocharged engine

ABSTRACT

This method for determining, in a turbocharged engine, the temperature of the exhaust gases downstream of the turbine of the turbocharger, comprises the following steps:
         determining the temperature upstream of the turbine of the turbocharger,   calculating a corrective term from engine operating parameters, and   determining the temperature downstream of the turbine of the turbocharger by subtracting the corrective term from the temperature upstream of the turbine of the turbocharger.

The present invention relates to a method for determining thetemperature before the inlet to a catalytic converter of a turbochargedengine.

In an engine fitted with a catalytic converter it is important to knowthe temperature at the catalytic converter in order not to destroy thelatter. This temperature is important for various functions: forprotecting the catalyst and its upstream oxygen probe, for detectingthat the upstream oxygen probe is ready, for heating the upstream oxygenprobe and for heating the catalyst.

In some engines, these four functions, or at least some of them, do notexist. In other engines, these functions are regulated in open loop. Itis also known for the control of these functions to take account ofparameters that are not so precise as the temperature at the inlet tothe catalytic converter.

In a normally-aspirated engine, it is known practice for the temperatureat the inlet and at the outlet of the catalytic converter to be modeled.The temperature of the engine exhaust leaving the engine is also knownby mapping.

The object of the present invention is therefore to provide a methodthat will enable the temperature at which the exhaust gases enter thecatalytic converter, that is to say the temperature downstream of theturbocharger, to be determined reliably in a turbocharged engine fittedwith a catalytic converter.

To this end, the invention proposes a method for determining, in aturbocharged engine, the temperature of the exhaust gases downstream ofthe turbine of the turbocharger, which comprises the following steps:

-   -   determining the temperature upstream of the turbine of the        turbocharger,    -   calculating a corrective term from engine operating parameters,        and    -   determining the temperature downstream of the turbine of the        turbocharger by subtracting the corrective term from the        temperature upstream of the turbine of the turbocharger.

This is something which is very simple to determine but, as has beendemonstrated, the temperatures determined using this method aretemperatures that are more or less consistent with those recorded usinga temperature probe in order to confirm this method.

The temperature upstream of the turbine of the turbocharger can bedetermined using a temperature probe but, in order to minimize the costof the corresponding engine, it is preferably obtained by modeling. Theperson skilled in the art knows how to model this temperature and suchmodeling is already used to prevent the turbocharger from overheating.This modeling is, for example, done by an electronic device built intothe engine control and management device. The corrective term can thenadvantageously be calculated by this same electronic device.

In a preferred embodiment, the corrective term is obtained first of allfrom a predetermined curve giving a temperature change as a function ofthe engine speed and of the mass air flow rate of air passing throughthe engine then by multiplying this temperature change by an adiabaticcompression factor.

The adiabatic compression factor is advantageously dependent on at leastone physical quantity chosen from the set comprising the engine exhaustpressure, the difference between this pressure and the external pressureand the opening of a waste gate of the turbocharger. In practice, justone quantity is sufficient to yield excellent results. In order then todefine the three-dimensional curve that gives the temperature changedependent on the engine speed and on the mass air flow rate, this enginespeed and the mass air flow rate are for example varied keeping thequantity/quantities on which the adiabatic compression factor isdependent constant.

The details and advantages of the present invention will become betterapparent from the description which follows given with reference to theattached diagrammatic drawing in which:

FIG. 1 schematically depicts the architecture of a turbocharged engine,and

FIG. 2 is a diagram to explain the way in which the method according tothe invention works.

FIG. 1 very diagrammatically depicts an air-supply and exhaust systemfor a turbocharged engine. This system supplies fresh air to an enginein which at least one piston 2 moves in a cylinder 4. The fresh airenters the cylinder 4 via an opening controlled by an inlet valve 6. Avalve 8 is provided for exhausting the burnt gases from the cylinder 4.

The air supply system depicted comprises, from the upstream directiondownstream, an air inlet 10, a mass air flow meter 12, a turbocharger14, a chamber known as an intercooler 16, a butterfly valve 18positioned in a pipe through which the air fed to the cylinders passesand able to alter the cross section for the flow of air through thispipe, and what is generally known as an inlet manifold 20. The inletvalves 6 are in direct communication with the inlet manifold 20.

The exhaust valves 8 are in direct communication with an exhaust pipe22. In order not to clutter the drawing, this exhaust pipe 22 isdepicted only where it leaves the cylinder and at the turbocharger 14.The latter comprises two turbines connected by a shaft. A first turbineis positioned in the exhaust pipe and its rotation is driven by theburnt gases leaving the cylinders 4 via the exhaust valves 8. The secondturbine, as mentioned earlier, is positioned in the engine air supplysystem and pressurizes the air in the intercooler 16. In theconventional way, a turbocharger waste gate 24 allows the turbinepositioned in the exhaust pipe 22 to be short-circuited.

On leaving the turbocharger, the exhaust gases pass through a catalyticconverter 26 before being discharged to the open air.

The method described hereinbelow can be used to determine thetemperature of the exhaust gases as they enter the catalytic converter26. As mentioned in the preamble, knowledge of this temperature isimportant to the operation of the catalytic converter 26. This catalyticconverter 26 contains an upstream oxygen probe (not depicted) whichprovides information to the engine management device in order to controlthe richness of the fuel/air mixture sent by the air supply system intothe cylinders 4. Knowledge of the temperature upstream of the catalyticconverter 26 and downstream of the turbocharger 14 allows the catalystand the upstream oxygen probe to be protected from excessively hightemperatures. When an excessively high temperature is detected, it ispossible to alter the engine supply in order to reduce the temperatureof the exhaust gases leaving the cylinders 4. It is also necessary,conversely, for the catalyst and the corresponding upstream oxygen probeto be at a relatively high temperature in order to be able to operatecorrectly. Knowledge of the temperature at the inlet to the catalyticconverter 26 therefore makes it possible to determine whether theupstream oxygen probe is ready and therefore whether the information itis supplying should be taken into consideration. It is also possible toanticipate heating the upstream oxygen probe and the catalyst wheretheir temperature is not high enough.

The person skilled in the art knows how, in a normally aspirated orturbocharged engine, to model the temperature in the exhaust pipe at theexit from the cylinders 4. Numerous parameters are used to determinethis temperature, these including, for example, but without beingexhaustive: the engine speed, the mass air flow rate, the richness ofthe fuel/air mixture sent to the cylinders, the ignition advance, etc.

The present invention proposes to calculate the temperature at the inletto the catalytic converter 26, that is to say at the outlet from theturbocharger, from the (modeled) temperature upstream of theturbocharger. In order to do this, it proposes to extract from the basicmap that determines the temperature before the turbocharger 14 a mapdependent on the engine speed and on the mass air flow rate of airpassing through the engine multiplied by an adiabatic compression factordependent on a parameter such as the exhaust pressure and/or the openingof the turbocharger waste gate 24.

FIG. 2 illustrates a diagram explaining how the temperature downstreamof the turbocharger 14 at the inlet to the catalytic converter 26 isdetermined according to the invention.

This FIG. 2 shows a three-dimensional curve depicted schematically in afirst window 28. An orthogonal frame of reference is also depictedschematically in this window 28. The curve schematically depicted givesa temperature change TC determined from the engine speed N and from themass air flow rate MAF measured by the flow meter 12. Thus, one axis ofthe frame of reference corresponds to the engine speed N, a second axiscorresponds to the mass air flow rate MAF while the third axis indicatesthe value of the temperature change TC.

Under the window 28 there is a second window 30 in which are depicted acurve and a two-axis orthogonal frame of reference. The abscissa axiscorresponds to one parameter while the ordinate axis corresponds to amultiplicative factor α. The parameter along the abscissa axis may bethe exhaust pressure PE measured in the exhaust pipe 22 at the exit fromthe cylinders 4. It may equally be the pressure difference between thisexhaust pressure PE and the atmospheric pressure outside the engine. Itmay finally be the opening (in degrees or as a percentage) of theturbocharger waste gate 24. This opening is termed WG in FIG. 2.

The temperature in the exhaust pipe 22 upstream of the catalyticconverter 26 is considered to adopt the value T_(upstream). Likewise,downstream of the turbocharger 14, the temperature adopts a valueT_(downstream). Thus ΔT=T_(upstream)−T_(downstream).

According to the present invention, it is considered that ΔT=α.TC.

Hence: T_(upstream)=T_(downstream)−α.TC.

In order to produce the map depicted schematically in the window 28, theparameter of the window 30 (WG, PE, P_(atm)−PE) is kept constant. Theengine speed and the mass air flow rate through the engine are thenvaried simultaneously in order to obtain a temperature change TC. FIG. 2schematically depicts the construction of two points on the map of thewindow 28. These points yield the values TC₀ and TC₁ when the pair (MAF,N) adopts the values (MAF₀, N₀) and (MAF₁, N₁) respectively. Thedetermined values TC₀ and TC₁ correspond to a turbocharger waste gate 24opening corresponding to a value WG₀.

Once this map has been produced, the curve in the window 30 is produced.To do this, the value of the parameter WG is then varied. FIG. 2schematically shows how to obtain two points on the curve in the window30 with WG values of WG₁ and WG₂. This then yields coefficients α₁ andα₂ respectively.

In order thus in a turbocharged engine to determine the temperature atthe inlet to the catalytic converter 26, the temperature upstream of theturbocharger 14 is first of all determined in the known way. Thisfunction is already known and carried out on certain engines. Then, as afunction of the engine speed N and of the mass air flow rate MAFmeasured by the flow meter 12, the value TC needs to be determined.Likewise, according to the variable WG, PE or P_(atm)−PE chosen, thecorresponding curve needs to be used to determine the corrective factorα. Multiplying the coefficient TC by α yields ΔT, the corrective termthat can be used in order immediately to determine the valueT_(downstream).

This method has been validated on engines and quite accurately yieldsthe temperature downstream of the turbocharger, at the inlet to thecatalytic converter 26. It is therefore possible in this way todetermine this temperature accurately without using a sensor.Furthermore, there is no need to employ additional electronic means inorder to determine this temperature. When a vehicle is equipped withmeans for determining the temperature upstream of the turbocharger,these same means can determine the temperature downstream of thisturbocharger using a method according to the invention. The additionalcost associated with determining this temperature at the inlet to thecatalytic converter 26 is therefore very low while at the same timeaffording great advantages as far as the life of the catalyst and of theupstream oxygen probe with which it is equipped is concerned.

In order to further illustrate the advantage afforded by determining thetemperature at the inlet to the catalytic converter 26, a numericalexample is given below. It is generally considered that the inlettemperature of exhaust gases entering the turbocharger must not exceedabout 1000° C. As far as the upstream oxygen probe is concerned, it ispreferable for its temperatures not to exceed around 750° C. Whentemperatures close to 1000° C. are obtained upstream of the exhaustturbine of the turbocharger 14, for example temperatures of 950° C.,temperatures that may be as high as 850° C. are found downstream of theturbocharger. When the oxygen probe is close to the turbochargerturbine, it is then necessary to instigate an enriching (with fuel) ofthe mixture burnt in order to lower the temperature at the inlet to thecatalytic converter 26. In engines of the prior art, no enrichment inorder to protect the turbine is performed because the limiting value of1000° C. is not achieved.

The present invention is not restricted to the embodiment of the methodthat has been described hereinabove by way of nonlimiting example. Italso relates to all embodiment variants within the competence of thoseskilled in the art within the scope of the claims that follow.

1. A method for modeling the exhaust gas temperatures in a turbochargedengine downstream of the turbine of the turbocharger (14), comprisingthe following steps: determining the temperature upstream of the turbineof the turbocharger (14), calculating a corrective term from engineoperating parameters, and determining the temperature downstream of theturbine of the turbocharger (14) by subtracting the corrective term fromthe temperature upstream of the turbine of the turbocharger (14),characterized in that the corrective term is obtained first of all froma predetermined curve giving a temperature change (TC) as a function ofthe engine speed (N) and of the mass air flow rate (MAF) of air passingthrough the engine then by multiplying this temperature change (TC) byan adiabatic compression factor (α).
 2. The method as claimed in claim1, characterized in that the adiabatic compression factor (α) isdependent on at least one physical quantity chosen from the setcomprising the engine exhaust pressure (PE), the difference between thispressure and the external pressure and the opening (WG) of a waste gateof the turbocharger (24).
 3. The method as claimed in claim 2,characterized in that, in order to define the three-dimensional curvethat gives the temperature change dependent on the engine speed (N) andon the mass air flow rate (MAF), this engine speed (N) and the mass airflow rate (MAF) are varied keeping the quantity/quantities on which theadiabatic compression factor (α) is dependent constant.